The present invention relates broadly to a torque motor apparatus, and in particular to an improved torque motor driver apparatus.
The state of the art of torque motor drivers is well represented and alleviated to some degree by the prior art apparatus and approaches which are contained in the following U.S. patents:
U.S. Pat. No. 3,868,554 issued to Konrad on Feb. 25, 1975; PA1 U.S. Pat. No. 4,531,081 issued to Liesegang on July 23, 1985; and PA1 U.S. Pat. No. 4,574,226 issued to Binder on Mar. 4, 1986.
The Konrad patent describes a current limit system for a DC motor control of the switching regulator or pulse control type which factors in both the mark-space ratio of the control, and the actual current through the motor. The maximum allowable motor current is thus controlled as a function of the mark-space ratio of a thyristor coupling the motor to a source of electrical energy.
The Liesegang patent is directed to an aircraft control surface servomotor control of the pulse width modulation type with precise motor current control for providing precise torque limiting measurements.
The Binder patent discusses the method and apparatus for controlling an electric motor, in particular a motor for driving hand tools, saws, and the like, the rotational speed of which is automatically reduced in no-load idling operation and increased when working under load to pre-determined speed values corresponding to desired working speeds. A current sensor picks up the current absorbed by the electric motor and causes the speed of the electric motor to be abruptly changed when a load condition has been detected.
The early prior art approach to torque motor control is shown in FIG. 1 which illustrates a typical circuit for a torque motor driver. The circuit is a typical voltage-to-current converter in which the current range varies from 0 to 100 milliamperes. The steady-state operating current (null current) is 50 milliamperes. Several shortcomings exist with the present prior art driver configuration. First, the driver operational amplifier must operate steady state at 50 milliamperes, which causes high power dissipation in the semiconductor driver. Second, if a cable short to ground from the high or low side of the torque motor (TM) occurs, the driver will saturate. This can result in high dissipation, overstress and damage to the driver and other circuit components. While the above-cited prior art patents are instructive, it is clear that a need still remains to provide a torque motor driver apparatus which solves the shortcomings of the prior art. The present invention is intended to satisfy that need.